This invention relates to packages of material wherein the contained material includes a substance subject to vaporization when the material is heated. More particularly, the invention relates to the cooking of materials such as foods in microwave ovens, and specifically to the cooperative roles of the food and the container during the cooking process.
It is widely known that microwave ovens are especially well adapted for cooking of convenience-type foods which can be cooked in a short period of time with minimal work on the part of the cook. It is also widely known that, in the markets served by microwave cooking, a substantial value is attached to convenience and time. Thus, manufacturers of disposable food service items have successfully adapted their products and introduced them as disposable cooking containers for use with microwave ovens. A typical disposable container has a paperboard tray containing the food, with a film lid sealed over the top of the tray. Such a container is economical to manufacture. It readily contains and protects the food.
In typical applications the package is kept in refrigerated storage, and is put in the microwave oven while still frozen. As heat is generated in the food in the microwave oven, the vapor pressure inside the sealed package rises with the increase in temperature. As the vapor pressure rises, it is important that the evolving vapor (and eventually steam) be released from the package by some venting means; or else the vapor pressure inside the package increases to the point where the package bursts, the burst normally occurring in the film lid.
As a means of vapor release, some packagers suggest that the film lid should be loosened before the package is put in the oven. Others suggest puncturing the lid before putting it in the oven. Still others provide punctures in the lid and cover it with an overwrap which is to be removed before putting it in the oven. Common to all of these products is the fact that the sealed integrity of the package is penetrated before the package is put in the oven. In a more expensive solution, a one-way valve may be built into the package.
In one practical design, wherein an economical package may be put into the oven while still sealed, a self-venting film lid is disclosed in U.S. Pat. No. 4,210,674, of common assignment herein. In that case, a material sensitive to microwave energy is printed on the lid film; and the absorbed microwave energy melts vent holes in the film. The printing of the absorbing material, however, may not always be acceptable.
Thus it is desirable to provide an alternate method of venting a package which is placed in a microwave oven in a sealed condition.